I. Field of the Invention
The current invention relates to communication. More particularly, the present invention relates to a novel method and apparatus for quick retransmission of signals in a communication system.
II. Description of the Related Art
In a communication system, a communication channel through which signals travel between transmitting and receiving terminals is subject to various factors, changing characteristics of the communication channel. In wireless communication systems these factors comprise, but are not limited to: fading, noise, interference from other terminals, and the like. Consequently, despite extensive error control coding, certain packets are missed or received erroneously at a receiving terminal. Unless defined differently, a packet is a unit of a signal comprising a preamble, a payload, and a quality metric. Therefore, Automatic Retransmission reQuest (ARQ) schemes are often used at the link layer of communication systems to detect missing or erroneously received packets at the receiving terminal, and request retransmission of these packets at the transmitting terminal. An example of an ARQ is a Radio Link Protocol (RLP). RLP is a class of error control protocols known as NAK-based ARQ protocols, which are well known in the art. One such RLP is described in TIA/EIA/IS-707-A.8, entitled xe2x80x9cDATA SERVICE OPTIONS FOR SPREAD SPECTRUM SYSTEMS: RADIO LINK PROTOCOL TYPE 2,xe2x80x9d hereinafter referred to as RLP2, and incorporated herein by reference.
Existing ARQ schemes achieve retransmission of missing or erroneously received packets by utilizing a sequence number unique to each packet. When a receiving terminal detects a packet with a sequence number higher than an expected sequence number, the receiving terminal declares packet(s) with sequence number(s) between the expected sequence number and the detected packet""s sequence number missing or erroneously received. The receiving terminal then sends a control message requesting retransmission of the missing packets to a transmitting terminal. Alternatively, the transmitting terminal may resend the packet after a certain timeout interval if the transmitting terminal has not received a positive acknowledgement from the receiving terminal.
Consequently, existing ARQ schemes cause a large delay between the first transmission of a packet and a subsequent retransmission. The ARQ does not declare a particular packet missing or erroneously received until the next packet, containing a sequence number higher then an expected sequence number is received or until the time outinterval expires. This delay results in a large variance in the end-to-end delay statistics, which has a further detrimental effect on the network throughput. Transport layer protocols such as the transport control protocol (TCP) implement a congestion control mechanism, which reduces the number of outstanding packets in a network based on the variance of the round-trip delay estimate. In effect, larger variance of delay results in a reduction of the amount of traffic that is admitted into the network and a subsequent reduction in throughput of a communication system.
One approach to reducing the delay and the delay""s variation is to avoid retransmissions by ensuring that the first transmission is received correctly with high probability. However, this approach requires a large amount of power, which in turn reduces throughput.
Based on the foregoing, there exists a need in the art for an ARQ mechanism with low retransmission delay.
The present invention is directed to a method and an apparatus for quick retransmission (QARQ) of signals in a communication system.
In accordance with one aspect of the invention, a receiving terminal determines a quality metric of a packet of received signal. The receiving terminal immediately sends a short acknowledgement (SA) to a transmitting terminal in accordance with the quality metric of the packet. If the quality metric indicates that the packet was incorrectly received, then the SA is termed negative acknowledgement (NAK); otherwise, the SA is termed positive acknowledgement (ACK) or acknowledgement.
In another aspect of the invention, there exists a determinable relationship between a particular packet and the SA; therefore, there is no need for the SA to contain an explicit indication as to which packet is to be retransmitted.
In accordance with another aspect of the invention, the SA is a bit of energy.
In accordance with another aspect of the invention, the transmitting terminal attempts retransmission of the packet a predetermined number of times.
In accordance with yet another aspect of the invention, a conventional, sequence-number-based ARQ is employed together with the QARQ scheme.